The evolution of the market of data storage memories indicates a growing need for ever-larger capacity, ranging from gigabytes to hundreds of gigabytes or even to terabytes. This evolution is driven, amongst others, by new data consuming applications such as multimedia and gaming. Flash memory technology, for example, which uses the shift in threshold voltage of a field effect transistor to indicate bit status, has so far been able to fulfil this scaling requirement, keeping a reasonable cost per bit. However it is expected that Flash memory technology will face severe scaling problems beyond the 45 nm technology node due to fundamental physical and/or cost limitations.
Resistive switching memories constitute replacement candidates, as their physical switching mechanism may not degrade with scaling. These types of memories comprise a resistor element that can be reversibly programmed in either a high or a low conductive state. Various materials such as transition metal oxides, organic semiconductors or organometallic semiconductors can be used to manufacture such resistor elements.
A resistive switching memory is based on the switching of the resistance. A material or device with switchable resistance value is placed between two electrodes. Switching is done by applying a voltage exceeding a threshold for changing the resistance (i.e. from low to high or vice versa). The memory cell may then be read by measuring the resistance value.
Resistive switching memories could potentially provide greater density, lower power usage, greater speed, and lower cost than Flash memory.
Resistive switching memories are being integrated using structures derived from the 1T/1R (one transistor/one resistor) and 1D/1R (one diode/one resistor) concept as used in dynamic RAM. The resistor element, comprising the resistive switching material, is stacked on top of a semiconductor device such as a MOS transistor, a bipolar transistor, or a diode, and is accessed through a bit-line. The resistor element is placed between metal lines or between the contact to the transistor and first metal level, typically within the back-end-of-line (BEOL) section of the integrated circuit.
The resistive memory device is integrated after the production of the transistors (i.e. after the front-end-of-line processes (FEOL)) and before the completion of the full interconnect stack (i.e. before the completion of the back-end-of-line processes (BEOL)).
PCT patent application WO2008/026081 discloses a method for manufacturing resistive switching devices. The resistive switching device comprises a bottom electrode, a top electrode and a layer of resistive switching material contacted by the bottom electrode and the top electrode. For forming the resistive switching device a dielectric layer is formed on a substrate, the substrate comprising the bottom electrode. In the dielectric layer a trench opening is formed so as to expose the bottom electrode. A resistive material is formed in the opening. The top electrode is formed on top of this resistive material.
There is a continuous need for a method to form a resistor element comprising a resistive switching layer, which method allows further scaling of resistor arrays.
There is also a need for a method to form a resistor element comprising a resistive switching layer, which would facilitate the integration of resistive switching materials in CMOS compatible process flows.